Recently, electronic technology has been remarkably developed, and various appliances have been made smaller and lighter. Along with the miniaturization and reduction in weight of electronic appliances, miniaturization and reduction in weight of batteries, serving as power sources of these electronic appliances, have been demanded. As batteries that have small volume and mass but are capable of providing large amounts of energy, non-aqueous electrolyte secondary batteries using lithium have been used. In addition, it has been proposed to use non-aqueous electrolyte secondary batteries as energy sources for hybrid cars, electric cars, and the like, and they have started to be put into practical use.
Generally, a non-aqueous electrolyte secondary battery has a positive electrode, a negative electrode, and a separator provided therebetween for insulating the positive electrode and the negative electrode. Conventionally, a porous film of a polyolefin-based polymer has been used as a separator used in a non-aqueous electrolyte secondary battery.
In non-aqueous electrolyte secondary batteries, charging and discharging are possible by ions (in the case of a lithium-ion secondary battery, lithium ions (Li+)) moving between a positive electrode and a negative electrode through a separator. Therefore, the separator is required to not inhibit ions from moving freely, and a porous film having a plurality of microscopic pores has been used as the separator.
In addition, the separator is required to have a so-called shutdown function. The shutdown function is a function that improves safety of the non-aqueous electrolyte secondary battery by, in the case where a tiny short circuit has occurred in the battery, inhibiting the movement of ions by blocking the holes in the part where the short circuit occurred in order to make the battery lose the function at that part. In the porous film of a polyolefin-based polymer, the shutdown function is achieved by, in the case where a tiny short circuit occurred in the battery, melting the part where the short circuit occurred by increasing the temperature and thereby blocking the holes.
However, when the shutdown function occurs, the separator shrinks, and as a result, secondary problems occur, such as the directly opposing positive electrode and negative electrode making contact and forming a short circuit.
A known method to solve such problems is to provide, on at least one face of the separator, a layer using a resin or the like having lower heat resistance than the separator.
For example, it has been proposed to provide, on at least one side of the separator, a surface layer containing polymer particles having a lower melting point than the separator and a binding agent (for example, see Patent Document 1).
Patent Document 1 discloses that due to the surface layer containing polymer particles having a lower melting point than the separator, the polymer particles having a lower melting point than the separator melt before the separator does when the battery internal temperature becomes high, and by forming a film on the separator surface, the pores of the separator, which is a porous film, can be blocked before the separator shrinks.
As another example, it has been proposed to provide a heat-resistant resin porous membrane containing from 0.1 to 5 wt % of fluorine resin microparticles on at least one side of the separator (for example see Patent Document 2).
Patent Document 2 discloses that the heat-resistant resin porous film has excellent abrasion resistance due to containing fluorine resin microparticles.